In the still important world of analogue electronics, much basic solid state circuitry can be traced back to the valve era. Developments are often optimised solid-state versions of circuits pioneered in the heyday of valves. For much of that period, there was intense interest in improving the characteristics of tunable LC oscillators: Hartley, Colpitts, Franklin, ECO, Gouriel Clapp, Seiler, Vackar oscillators all had their enthusiastic followers - and all have since appeared in solid state guise.

My interest in one largely forgotten form of oscillator, first described by H E Harris in 1951, was aroused by the Dutch journal Electron (December 1990) which reprinted from its February 1958 issue an article by J J van Gelderen, PA0VGR describing a form of Harris VFO, using a 6J6 valve.

The Electron item led me to Radio & TV News, June 1957 'A high-stability oscillator circuit' by Robert J Ropes; this in turn referred me to J K Clapp's 'Frequency stable LC oscillators' (Proc IRE, Aug 54) providing a detailed survey of a number of LC oscillators and to the original presentation of the cathode-follower family of oscillators by H E Harris in connection with an article on a 'Simplified Q Multiplier' (Electronics, May 51,).

In the Harris oscillators (Fig 1) use is made of the fact that while a cathode-follower has less than unity voltage gain, it has power gain, good phase regulation and presents a very high impedance at its grid. With a valve having a low grid-cathode capacitance this means that full advantage can be taken of the high-Q LC tank circuit with good isolation between the i/p and o/p circuits. In other words, stability is largely determined by the LC tuned circuit.

In effect, a portion of the cathode-follower output is stepped up by passive components and fed back to the grid of the valve to provide the positive feedback necessary to sustain oscillation. In the original application as a (regenerative) Q-Multiplier, it gave controllable selectivity of a very high order with excellent stability: Fig 2. The later applications, developed by Ropes and PA0VGR, showed that the basic Harris ideas could usefully be applied to HF VFOs for amateur radio, providing a stable output that can remain constant over wide variations of the LC ratio, a characteristic that cannot be achieved with the conventional high-C Colpitts oscillator.

R J Ropes dubbed his version a 'Class A Colpitts' (more precisely Class AB) but pointing out that it differs radically from the conventional Colpitts oscillator: "Since the oscillator operates in Class AB, no grid current flows during any part of the oscillatory cycle, there is no 'grid-leak' capacitor and no grid-bias voltage is produced by grid-current flow, as is the usual case in a Class C oscillator."

He also noted that: "As pointed out by Clapp, the frequency coefficient of an oscillator is independent of the LC ratio if the operation of the circuit is linear, that is Class A, AB or B. Since Class A operation of an oscillator is, for all practical purposes, impractical, Class AB or B operation must be used to give the necessary linearity of oscillation."

As with other Colpitts, Hartley, ECO type oscillators, maximum stability is obtained when the capacitive divider or inductive tapping is as close as possible to the earthy-side of the tank circuit while still providing the necessary feedback. With capacitive tapping, this implies making the lower capacitor of higher value than the capacitor connected to the grid. It should be noted that R1 (Fig 3) provides the positive feedback and is not intended to pass DC in the manner of a grid leak. For Q-multipliers, the degree of positive feedback and hence the onset of oscillation can bu controlled by making this resistor variable. Active devices should have reasonably high gain and low input capacitance. Ropes used a 6J6 twin-triode with both sections in parallel. PA0VGR's circuit shows only a single section of the 6J6.

Fig 3: A Harris cathode-follower oscillator used for a 3.5MHz VFO by R Ropes and adopted aiso by PA0VGR. The Ropes design used both sections of the 6J6 dual-triode with 150V (regulated) HT (anode current 1.2mA) covering 3.5 to 4MHz (approximately 90% rotation of the 25pF tuning capacitor) with L 21µH (24 turns No 22, enamelled copper wire closewound on 1.25" diameter former).